Furnace components such as radiant burner tubes must be able to withstand high temperatures and corrosive environments in industrial heat-treating and in aluminum melting furnaces. Commercially-available burner tubes operate in the range from about 900.degree. C. to about 1250.degree. C. and are generally metal alloy tubes, ceramic monolith tubes, and ceramic composite tubes. Of the first type, nickel-based superalloy metal tubes are commonly used, but are limited to the lower temperature range of 900.degree.-1100.degree. C. Of the second type, monolithic silicon carbide radiant burner tubes are commonly used and generally have temperature capabilities up to about 1250.degree. C. but suffer from the brittle failure problems typical of monolithic ceramic shapes. Furnace components, used in very high temperatures and in corrosive environments, require a special selection of materials to avoid chemical and mechanical disintegration of the ceramic. Ceramic-ceramic composites, using ceramic fibers and cloths as reinforcements in a ceramic matrix, are the third type of tube and are frequently the most desirable choice for use in high temperature, chemically-corrosive environments.
One type of commercially-available radiant burner tube is produced under the designation SICONEX(TM) Fiber-Reinforced Ceramic, and is commercially available from Minnesota Mining and Manufacturing Company, St. Paul, Minn. SICONEX(TM) Fiber-Reinforced Ceramic is a ceramic-ceramic composite comprising aluminoborosilicate fibers in a silicon carbide matrix. SICONEX(TM) Fiber-Reinforced Ceramic is prepared by first forming a tube or other shape of NEXTEL(TM) aluminoborosilicate ceramic fibers (commercially available from Minnesota Mining and Manufacturing Company, St. Paul, Minn.) by braiding, weaving, or filament-winding the ceramic fibers. The ceramic fiber shape is treated with a phenolic resin to rigidize it, and then coated via chemical vapor deposition at temperatures ranging from 900.degree. to 1200.degree. C. to produce a relatively impermeable, chemically-resistant matrix of a refractory material such as beta-silicon carbide. The resultant rigid ceramic composite is then useful at high temperatures and in corrosive environments.
However, the utility of these materials as furnace components can, depending on the degree of their permeability to gases, be somewhat limited. Ceramic-ceramic composites such as SICONEX(TM) are comprised of relatively open networks of fibers and can remain permeable to gases, even after extensive overcoating with a ceramic (e.g., silicon carbide) layer.
While there have been many approaches to sealing ceramic composite surfaces, these attempts have not been coupled with sufficient matching of chemical, thermal, and mechanical properties of the coating to achieve adequate thermal and chemical behavior at extreme temperatures and reaction conditions. Thermal expansion coefficient matching is especially critical due to the elevated temperatures of use and repeated thermal cycling in typical furnace applications.
Previous work in this field generally is directed to coating, sealing, or adhering refractory materials. U.S. Pat. No. 4,358,500 and related U.S. Pat. No. 4,563,219 describe a composition for bonding refractory materials to a porous base fabric such as fiberglass, using a coating comprised of colloidal silica, monoaluminum phosphate, and aluminum chlorohydrate. The coating provides heat and flame protection to the fiberglass fabric.
U.S. Pat. No. 4,507,355 describes an inorganic binder prepared from colloidal silica, monoaluminum phosphate, aluminum chlorohydrate and a catalyst of alkyl tin halide. This mixture is applied to the preferred substrate fiberglass to form a heat-resistant fabric.
U.S. Pat. No. 4,592,966 teaches a method of strengthening a substrate (fiberglass or fiberglass composites) by impregnating the substrate with, for example, aluminum or magnesium phosphate, magnesium oxide, or wollastonite, and a non-reactive phosphate. This is described as a cement which lends strength to the fiber substrate.
U.S. Pat. No. 4,650,775 describes a thermally-bonded fibrous product wherein aluminosilicate fibers are bonded together with silica powder and boron nitride powder. These mixtures can be formed into different shapes and used as diesel soot filters, kiln furniture, combustor liners, and burner tubes.
U.S. Pat. No. 4,711,666 and related U.S. Pat. No. 4,769,074 describe an oxidation prevention coating for graphite. A binder/suspension of colloidal silica, mono-aluminum phosphate and ethyl alcohol is applied to a graphite surface and prevents oxidation during heat cycling.
U.S. Pat. No. 4,861,410 describes a method of joining a metal oxide ceramic body such as alumina with a paste of a sol of a metal oxide, aluminum nitrate, and silicon carbide. This method is used to repair cracks in ceramic materials and to permanently join ceramic structures together.
Silicon carbide-ceramic fiber composites would benefit greatly from a coating that would protect the composites in high temperature and corrosive environments. To be most effective for high temperature uses, the coating needs to match the thermal expansion coefficient of the composite. In uses which require minimal transfer of gases through the walls, the coating needs to reduce the permeability of the silicon carbide-ceramic fiber composite. A further need in this field is the ability to adjoin ceramic composite pieces together or to patch holes in the composite articles.
To date, there has not been a coating composition which matches the thermal expansion coefficient of an aluminoborosilicate fiber-silicon carbide-coated composite under high temperature conditions, limits gas permeability and can be used to adjoin the aforementioned composites together. The increased use of ceramic composites in high temperature and corrosive environments creates a need for a coating composition with the above attributes.